A placebo-controlled, double-blind, crossover trial will be conducted providing study subjects either hydrocodone/acetaminophen 30mg/975mg, sustained release morphine 45mg or placebo on separate GCRC visits. A long acting comparator (slow-release morphine sulfate 45 mg) will be chosen because of its putative equianalgesic effects to the dose of hydrocodone (30 mg) selected. Subjects will participate in the three sessions at the UC Davis/Mather Medical Center General Clinical Research Center (GCRC) at intervals of 7-10 days. Sessions will be approximately 360 min in duration. Subjects will receive either hydrocodone/acetaminophen or sustained release morphine around-the-clock for 7-10 days prior to the experimental session. At each experimental session, an assessment of abuse liability will be completed before the intake of medications, as well as at 0, 60, 120, 180, 240 minutes after the ingestion of the study medication.